The problems associated with the lubrication of moving parts, such as in machinery, are well known to those skilled in the art. For example, in the lubrication of transmissions, proper fluid viscosity at both low and high temperatures is essential to successful operation. Good low temperature fluidity facilitates cold weather starting and ensures that the hydraulic control system will properly "shift gears". High viscosity at elevated temperatures ensures pumpability and the satisfactory functioning of converters, valves, clutches, gears and bearings.
In the operation of hydraulic fluid systems, proper fluid viscosity at both low and high temperatures is essential to successful operation and is well known in the art.
These conflicting fluidity requirements call for a product that exhibits the following characteristics:
(A) high temperature viscosity retention, PA1 (B) low temperature fluidity, and PA1 (C) shear stability. PA1 (A) a base fluid, and PA1 (B) a sulfur and phosphorus containing low molecular weight or non-polymeric viscosity modifying composition.
In order to prepare lubricants having these characteristics, it has become common practice to add a variety of chemicals to the oil. For example, in order to meet the viscosity requirements, compositions have been added to the oils which are characterized by a relatively small change in the viscosity of the oil with changing temperature. Such oils are commonly graded according to the viscosities at low (e.g., 0.degree. F.) and at high temperatures (e.g., 210.degree. F.) according to SAE standards. As a result of the incorporation of such additives, the lubricating oils are often referred to as being "multigrade". In terms of widely accepted concepts such multigrade lubricants have the desirable properties of being able to function immediately, though cold, upon being put into service, and to continue to function satisfactorily as they become heated during operation.
Although chemical compositions have been developed which improve the high viscosity characteristics of lubricating oil, it is often desirable to further improve the low temperature characteristics by including compositions which function as fluidity modifiers at low temperatures. Fluidity modifiers are capable of lowering the viscosity of a lubricating oil at low temperatures generally by retarding the formation of undesirable network of microcrystalline wax substances.
In addition to the above improvements, it is desirable, if not necessary, that the lubricating compositions especially designed for use as transmission fluids and hydraulic fluids exhibit shear stability. Shear stability means that the lubricating oils will not degrade or lose their desirable viscosity characteristics as a result of the shearing forces encountered during their use. Lubricating oil compositions exhibiting desirable shear stability will be found generally to have the viscosity within 85-95% of their original viscosity after a number of hours, (e.g., 100 hours) of service. It has been recognized that many ordinary viscosity index improvers commonly added to crankcase lubricating oils, such as high molecular weight polyisobutylene and polyacrylates, do not possess the desired shear stability for use in improving the viscosity characteristics of transmission fluids and hydraulic fluids.
It now has been found that multigrade lubricants exhibiting improved shear stability and low temperature viscosity performance can be formulated by utilizing the compositions of the present invention. The compositions of this invention are particularly useful as performance chemicals and rheology control agents in lubricating fluids such as hydraulic fluids, transmission fluids and crankcase fluids.